Add markdown linting

[jsumners]

This PR adds a workflow that will check the line length within Markdown documents in this project. It's possible we could migrate this to the common workflows repo, but I'm not clear if that is possible given how the tools in this workflow work. Anyone is welcome to do their own testing and move it if they determine it viable.

See https://github.com/stupid-org/md-lint/pull/1/files for an example of what a failed check looks like.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

This setup only runs in CI. I think we should be able to run locally too. I think remark might offer a better toolchain but I'm ok with anything I can run locally as well.

(In Node.js TSC we use https://github.com/nodejs/TSC/blob/main/package.json#L7)

[mcollina]

lgtm

[Fdawgs]

Had to remove it as a Required workflow from the main branch protection rules, see #4005 (comment)

[jsumners]

It's a really basic implementation. It should really be implemented as "do markdown changes exist? yes: run check, no: return green".

[github-actions]

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